1. Field of the Invention
The present invention relates generally to operation of bi-stable and mono-stable magnetic latching mechanisms, and more particularly to a device, method, and system for bi-stable and mono-stable magnetic latching mechanisms in safety related applications such as automotive brake systems and control mechanisms for the power industry, nuclear and conventional, that utilize permanent magnetic latching devices in control and safety systems.
2. Background of the Invention
The new generation of pulse operated magnetic latching devices require a bi-polar pulse in order for the magnetic latching device to operate as intended or designed. This divergence from traditional driving method of constant powered devices has caused or created problems in finding method(s) of effectively operating this generation of magnetic devices, especially units requiring substantial power operating in operationally critical environments. These pulse operated short duty cycle devices generate more xe2x80x98switching/change-over forcexe2x80x99 than their constant powered counter-parts, but lack the user confidence of the constant powered devices.
Manufacturers and users of these new magnetic latching devices have learned that just providing a pulse to the unit with no position indicator or operations indicator is not sufficient to meet today""s safety requirements and provide the operators or other users with the assurance of correct operation. The continuously powered devices, which are being replaced by the new generation of magnetic latching devices, have an inherent built-in safety mechanism and operational confidence lacking in the new magnetic latching designs.
Several manufacturers have attempted to address these problems by adding switches, modifying the magnetic circuitry to cause closure of a magnetic switch at one position, or developing electronics to monitor position by determining induction value of the magnetic assembly or visual indicators.
Another approach used by manufacturers and users is to over drive the magnetic latching mechanism by providing a higher voltage pulse of longer duration than required in order to establish an operating confidence level with the magnetic latching device. This causes heating of the brake or clutch, damage to the winding installation, and excessive power consumption, all of which the design was supposed to overcome.
The automotive environment does not make wide use of bi-polar power drivers that are required to operate these medium to high power driver requirements. (There appear to be no, or at least very few, high power, 30A, Double Pole Double Throw relays for automotive applications.) The industrial environment provides a better source for high power double pole double throw relays than the automotive industry, but lacks an economical method of providing the bi-polar electrical pulse necessary to correctly drive or operate the new generation of magnetic latching devices and mechanisms.
Various sources provide background information relating to different aspects or components of the invention. For example, patents and reference data books provide background information related to the present invention.
The following sources provide information related to peak detectors: (1) National Semiconductor Data Acquisition Databook, Operational Amplifier Databook, Power ICs Databook, and Application Specific Analog Products Databook; (2) Texas Instruments; (3) Fairchild; (4) Encyclopedia of Electronic Circuits, vols. 1-6; (5) Heath/Zenith Continuing Education; Electronic Technology Series; Operational Amplifiers; (6) Electronics Circuit Manual; (7) Linear Technology, 1994 Linear Databook, volume III; and (8) Sourcebook of Electronic Circuits.
Several references, mainly patent documents, provide background information related to position sensors and methods of determining position of an electromechanical device. Generally, these devices range from a modified magnetic circuit that incorporates a magnetic flux sensor to alternating current frequency determination based upon inductive value of the magnetic assembly to physical position indicators.
U.S. Pat. No. 3,089,064 to De Bennetot, entitled Combined Permanent Magnet and Electromagnet, which issued May 7, 1963, shows a Double Pole Double Throw (DPDT) switch controlling the operation of a permanent magnet assembly, which consists of a combination of parallel permanent and electromagnets.
U.S. Pat. No. 3,428,867 to M. C. Becker, entitled Method and Apparatus for Controlling the Useful Magnetornotive Force Of A Permanent Magnet, teaches use of a DPDT switch to control the flow of electrical current in a device using both permanent and electromagnets in a series magnet circuit.
U.S. Pat. No. 3,789,876 to Calvin E. Kempton and Robert H. Reinicke teaches a method of using an alternating current to determine the position of a solenoid armature with any mechanical connections to the armature (solenoid valve with electronic position indicator).
U.S. Pat. No. 4,004,258 to Kurt Arnold teaches a method of modifying the magnetic circuit path to create a flux gap in which a magnetic sensor is used to determine the position of an armature (position indicating latching solenoid).
U.S. Pat. No. 4,059,844 to John W. Stewart demonstrates methods of controlling current to a solenoid (solenoid driver circuit). U.S. Pat. No. 4,262,320 to Lee F. Herron shows a method of operating latching solenoids using an H-switching configuration (H-switch configuration for controlling latching solenoids).
U.S. Pat. No. 4,321,946 to Louis B. Paulos uses a differentiating network to monitor the current flow through a current sensing resistor to control a dual battery driver operating a monostable spring return solenoid to provide visual indicators (armature position monitoring and control device).
U.S. Pat. No. 4,341,241 to Joseph W. Baker uses a mechanical switch to indicate the position of a armature in a valve mechanism (position indicating valve means).
U.S. Pat. No. 4,490,771 to Siefried Huber and Manfred Merkator teaches a method of using an electrical control mechanism to use a portion of the electrical sine wave from a power line to operate a solenoid (control circuit arrangement for an electromagnetically operated power tool).
U.S. Pat. No. 4,620,173 to Robert B. O""Brien shows a method of using magnetic reed switches to show the position of a permanent magnet attached to a solenoid plunger (latching magnetic actuator).
U.S. Pat. No. 4,631,627 to Ronald E. Morgan teaches a method of controlling a relay which operates a motor (impulse operated relay system).
U.S. Pat. No. 4,733,212 to Arthur V. Goodwin, titled Pulse Latching Solenoid, shows a mechanical approach to determining the position of a solenoid armature.
U.S. Pat. No. 4,810,952 to Burton E. Cohen teaches a power delivery method for operating a fastening machine solenoid (circuitry and method for controlling power to fastener machine solenoid).
U.S. Pat. No. 4,810,964 contains a method and apparatus for measuring the distance between a measuring transducer and an opposing surface, particularly with paper pulp equipment.
U.S. Pat. No. 4,950,985 to Thomas Voss et al. teaches a method of determining the position of an armature in a magnetic system by using an alternating current (apparatus for measuring electromagnetic values of a coil, in particular for measuring the position of armature of a coil/armature magnetic system).
U.S. Pat. No. 5,121,018 to Stephen Z. Oldakowski, titled Latching Brake Using Permanent Magnet, teaches the use of parallel electromagnets and permanent magnets in a latching environment to operate a spring applied magnetic release brake operated by a bi-polar pulse.
U.S. Pat. No. 5,632,468 to Ivar Schoenmeyr uses a conventional power supply that rectifies and filters alternating current to provide direct current source of power allowing a solid state switch means to operate a solenoid (Control circuit for solenoid valve).
U.S. Pat. No. 4,970,622 to Josef Bilchl teaches a method of using the slope of the DC charging current energizing an electromagnet to develop an activation frequency of the magnetic assembly (method and apparatus for controlling the operation of an electromagnet). This frequency is compared to a predetermined value to provide an indication of the position of the armature.
U.S. Pat. No. 5,185,542 to Edward D. Lazorchak, titled Electromagnetic Pulse Operated Bi-Stable Brake, shows a series permanent magnet and electromagnet operating a spring applied magnetic released pulse operated brake.
U.S. Pat. 5,196,983 to Paul B. Stumpf teaches a method of using alternating current to determine the position of a solenoid plunger (solenoid engagement sensing circuit).
U.S. Pat. No. 5,241,218 to Michael Page teaches a method of detecting the current dip caused by a solenoid plunger, armature, seating to detect correct operation of a solenoid. The electronic detection circuit activates a single visual indicator, light emitting diode (LED) (Armature movement detector circuit).
U.S. Pat. No. 5,250,883 to Tadashi Okada teaches a motor control circuitry for starting and stopping a multi-speed motor (motor drive control apparatus).
U.S. Pat. No. 5,347,419 to LaVerrie A. Caron, et al., shows a digital approach of controlling the operating current to a solenoid (current limiting solenoid driver).
U.S. Pat. No. 5,443,132 to James H. Arnold, entitled Magnetic Latching Mechanism and Method Particularly for Brakes, teaches a method of manipulating magnetic field at the operating faces of pole pieces by alternately canceling the magnetic field on one operating face while enforcing the magnetic field on the other operating face by the same amount of magnet energy.
U.S. Pat. No. 5,583,434 to John C. Moyers, et al., teaches a method of injecting an alternating current into the direct current operating circuit of a DC solenoid to determine the position of the solenoid (method and apparatus for monitoring armature position in direct-current solenoids).
U.S. Pat. No. 5,701,109 to Peter Ulrik Poulsen demonstrates a unique method of using buck/boost magnetic principles to operate a current sensing relay used for inductive loads that does not have the in-rush currents associated with it) motors (current sensing relay).
Other patents of relevant to background of the present invention include the following: U.S. Pat. No. 3,740,615 relates to an activating and confirming device for printing electromagnets. U.S. Pat. No. 3,854,695 describes an electromagnet control apparatus. U.S. Pat. No. 4,004,258 relates to a position indicating latching solenoid. U.S. Pat. No. 4,041,546 describes a solenoid driver circuit. U.S. Pat. No. 4,112,365 is for a position detecting system.
The following patents provide additional background information. U.S. Pat. No. 4,295,111 (low temperature latching solenoid); U.S. Pat. No. 4,295,177 (control circuits for solenoids); U.S. Pat. No. 4,453,652 (controlled current solenoid driver circuit); U.S. Pat. No. 4,680,667 (solenoid driver control unit); U.S. Pat. No. 4,690,168 (valve actuator position indicating system); U.S. Pat. No. 4,729,056 (solenoid driver control circuit with initial boost voltage); U.S. Pat. No. 4,533,890 (permanent magnet bi-stable solenoid actuator); U.S. Pat. No. 4,749,891 (non-linear electromagnetic vibration device); U.S. Pat. No. 4,757,418 (solenoid driver circuit); U.S. Pat. No. 4,797,779 (pulsed power supply); U.S. Pat. No. 4,809,742 (control valve assembly including valve position sensor); U.S. Pat. No. 4,845,420 (drive circuit device for inductive load); U.S. Pat. No. 4,878,147 (electromagnetic coil drive device); U.S. Pat. No. 4,907,901 (method and apparatus for measuring displacement of a moveable member of an electromagnetic device by using perturbations in the device""s energizing current); U.S. Pat. No. 4,950,987 (magneto-inductive sensor for performing tactile and proximity sensing); U.S. Pat. No. 4,953,590 (electromagnetic directional control valve); U.S. Pat. No. 4,980,793 (open loop control of solenoid coil driver); U.S. Pat. No. 5,032,812 (solenoid activator having a magnetic flux sensor); U.S. Pat. No. 5,045,786 (circuit for measuring a variable inductance connected in series with a fixed inductance); U.S. Pat. No. 5,115,193 (inductive linear displacement transducer and temperature-compensating signal processor); U.S. Pat. No. 5,180,978 (proximity sensor with reduced temperature sensitivity using AC and DC energy); U.S. Pat. No. 5,218,308 (sensor for and method of detecting the position of a piston inside the cylinder of a dashpot); U.S. Pat. No. 5,250,884 (drive controlling apparatus); U.S. Pat. No. 5,258,669 (current sense amplifier circuit); U.S. Pat. No. 5,270,900 (solenoid response detector); and U.S. Pat. No. 5,289,131 (circuit configuration for monitoring an electromagnetically actuated device, in particular an electromagnetic clutch).
Other patents containing additional background information include U.S. Pat. No. 5,404,303 (solenoid current driver circuit); U.S. Pat. No. 5,422,593 (current-limiting circuit); U.S. Pat. No. 5,422,780 (solenoid drive circuit); U.S. Pat. No. 5,424,637 (method and apparatus for determining the position of an armature in an electromagnetic actuator using observer theory); U.S. Pat. No. 5,428,496 (electronic switching arrangement); U.S. Pat. No. 5,438,489 (solenoid driver circuit and diagnostics); U.S. Pat. No. 5,450,276 (electromagnetic switch device); U.S. Pat. No. 5,457,364 (bridge motor driver with short-circuit protection and motor-current limiting feature); U.S. Pat. No. 5,463,263 (permanent magnet control means); U.S. Pat. No. 5,470,043 (magnetic latching solenoid); U.S. Pat. No. 5,481,237 (solenoid valve with electrical connection elements and integrated safety devices); U.S. Pat. No. 5,497,093 (method and apparatus for diagnosing a twin-coil, bi-stable, magnetically latched solenoid); U.S. Pat. No. 5,529,281 (dual-latching solenoid-actuated valve assembly); U.S. Pat. No. 5,541,806 (dual current sensing driver circuit with switching energization and flyback current paths); U.S. Pat. No. 5,594,384 (enhanced peak detector); and U.S. Pat. No. 5,657,002 (resettable latching indicator).
The advent of pulse operated bi-stable and mono-stable magnetic latching devices is demonstrated by the related art. However, an important shortcoming of the art is a cost effective device, system, and method of operating these devices, along with a device, system, and method of determining if the magnetic latching mechanism actually performed the requested task (i.e., whether it actually applied or released); a device, system, and method to determine the state (position) of the magnetic latching mechanism; and a device, system, and method of determining the operating condition of the electronics and magnetic latching mechanism.
It is an object of the present invention to provide a cost effective and reliable mono/bi-stable magnetic latching device for an automotive application, such as an electromagnetic/electrical parking brake, and a method and system of operation of the device.
It is a further object of the present invention to provide a device, method, and system for determining if the magnetic latching mechanism is operating as intended. It is a further object of the present invention to provide a device, method, and system for determining if the magnetic latching mechanism is latching in the position chosen. It is a further object of the present invention to provide a device, method, and system for determining the operational status of the magnetic latching mechanism and the associate electronics.
It is a further object of the present invention to provide a device, method, and system for determining a manual means of operating the magnetic latching mechanism in the event of electronics failure, in order to establish operator/user confidence in this new generation of brakes, clutches, and valves.
Since this invention takes the first comprehensive look at the requirement of operating a magnetic latching device in a safety related application, it is a further object of the present invention to set the standard and acceptance level for mechanisms that operate magnetic latching devices.
The present invention provides a novel arrangement and use of circuit elements to perform the complex task of operating a magnetic latching mechanism in a safety related application, such as a parking brake system. The present invention includes use of a microcontroller based electronic design that contains a secondary operating system, and a manual over-ride system. In combination with the primary operating system, these secondary and manual over-ride systems provide an aerospace design level of reliability for the present invention. In addition, reliability is further assured by the use of a parking brake unsafe indicator on the dash and other components to provide the user with assurance of proper operation and other information about the system functioning, including indication of transitioning of the magnetic latching device.
The present invention, which is referred to as the xe2x80x9cLatch-Up Detector and Electrical Driver for Magnetically Biased/Latched Mechanisms and the Likexe2x80x9d and alternatively as the xe2x80x9cLatch-Up Detector/Driverxe2x80x9d, relates to the operation of bi-stable and mono-stable magnetic latching mechanisms in safety related applications, such as Automotive Parking Brake systems such as described in U.S. Pat. No. 5,443,132 (Arnold; Magnetic Latch Mechanism And Method Particularly For Brakes; issued Aug. 22, 1995) and control mechanisms for the power industry, nuclear and conventional, that utilize permanent magnetic latching devices in control and safety systems. The present invention is also applicable where correct operation on of a magnetic latching mechanism is critical or otherwise crucial to operation of a more complex mechanism.
The present invention includes the electrical and electronic control circuitry and other elements that drive and operate mono-stable and bi-stable brakes, clutches, and valves of spring applied, magnetically applied, or magnetic activators types in both full electronic drive modes and manual electrical modes that do not use any of advanced electronics for operation. In an embodiment of the present invention, these mechanisms are operable in full automatic operating mode with no user initiation required (computer controlled), in semi-automatic operating mode with only some user initiation required (partial computer controlled), or in a full manual operating mode, which requires full user initiated operation (user/operator control).
The Latch-Up Detector/Driver electronics include the following: 1) an operating algorithm that increases the operational effectiveness of magnetic latching devices; 2) cost efficient electrical drive circuitry that provides a bi-polar pulse to efficiently operate pulse operated magnetic devices; 3) electronic circuitry to detect latch-up that occurs when the ElectroMagnetic Assembly (EMA) within the driven mechanism changes state (changes from one position to another); 4) internal monitoring circuits that inform the operator or other user of the operating status of both the electronics and external driven mechanism; 5) an isolation resistance monitor to check the isolation resistance between the electrical winding of the electromagnets and the magnetic pole structure on which the electromagnets are wound; 6) performance monitors that monitor the time required for the magnetic assembly to change state; 7) position determining circuitry that determines the physical position of the driven mechanism; 8) an unscheduled event detector to monitor the EMA for a change in position created by such events as vibration or shock to the driven element; 9) Unswitched, Switched, and Enabled power supplies that provide the necessary power to operate the various analog and digital components contained within the mechanism; 10) voltage monitors that provide an indication of an out of tolerance voltage condition; 11) a Manual Over-Ride System that allows a method of Applying/Releasing the brake that is independent of the electronics; and 12) an Unsafe Parking Brake Indicator and associated logic that informs the driver or other user of the operating status of the parking brake system and also functions as a backup operating system in the vent of microcontroller failure.
The following features are included in an embodiment of the present invention. An Analog-to-Digital Converter (ADC) is used as a diagnostic tool in determining the operating condition of both the electronics and the external remotely operated magnetic latching mechanism. An Apply-Release Switch Operator provides introduction of the initiate fault circuitry and residual switch position memory. An Audio Enunciator is provided, which includes use of a single microcontroller 1/0 port to control the operation of an audio warning device and the ability to have two intensity levels from the single information channel. A Bi-Polar Pulse Driver is used for a xe2x80x98hybridxe2x80x99 electrical drive using a combination of mechanical relays and solid state switches to control the operation of a bi-directional device by including the capability to generate a bi-polar signal. Also included with the Bi-Polar Pulse Driver is the use of a tapped, or split, current sensing resistor to provide a differential current flow indication. A Brake/Clutch Position Sensor using a Direct Current Pulse electrically determines the position of an armature in a bi-stable magnetic latching device. The invention also includes a Brake/Clutch Status Indicator containing the xe2x80x98Parking Brake UnSafexe2x80x99 logic unit and indicator. The bi-polar light emitting diode (LED) driver provides individual current limiting resistors for each color of the bi-colored LED""s, allowing the driving circuit to be optimized for each color, and the Electronics Control Unit (ECU) includes use of the multiplexed and microcontroller tri-state output to increase the effective number of 1/0 ports available.
The invention further includes a Fault Monitor for the use of a differential signal pair to set and reset a latching relay and control the illumination of an indicator. This feature also includes use of a circuit to record and use the residual fault indication produced by the last displayed fault indication. An operating algorithm that calculates the operating temperature of the EMA and that uses the results to control the amount of time allowed between engagements is also included with the Fault Monitor. This operating algorithm prevents heat build-up from damaging the magnetic assembly. An Isolation Resistance Monitor is provided for the use and operation of an in-line and on-line isolation resistance monitor to determine the operational status of an external electrical assembly. A Latch-Up Detector is used that includes a combination of gated differential amplifiers, shunted peak detectors, inverters, and differentials to produce a bi-polar latch-up detector.
The present invention also uses a synthetic signal to check the operation of the latch-up detector and includes integration of a Manual Over-Ride system into a pure electronic design. The Manual Over-Ride System uses the combination of mechanical and electronic system to provide enhanced operational areas (including an unscheduled event detector).
The present invention further includes an Over Voltage Detector and a Position Memory that uses a microcontroller signal pair as a sensor and a control element for sensing the change in status. The Position Memory allows use of an override setting, and control of the illumination of an indicator by using the same pair of signal lines. An embodiment of the present invention further includes a Service Brake Interface, which allows a parking-brake-by-wire design, and a Transmission Interface that allows use of the transmission range select information in the operating program of a parking-brake-by-wire system. The present invention also includes a Power Supply, a Reference Voltage Generator, an Under Voltage Detector, a Voltage Monitor, and a Serial Number, Electronic. Other features of an embodiment of the present invention include an UnScheduled Event Detector that is able to detect the unscheduled event, and that allows the unpowered changing of the latch state of a bi/mono stable magnetic device and resetting the device back to its original state. A Wave Form Generator is provided for checking the operation of the latch-up detector/driver circuitry with the use of a synthetic signal.
An embodiment of the present invention thus uses a cost effective combination of solid-state and mechanical components and relays, to form a hybrid electrical drive unit that generates a bi-polar electrical pulse in a cost effective manner. Electronic circuits monitor the operating current to detect the xe2x80x98dipxe2x80x99 caused by the xe2x80x98armaturexe2x80x99 changing from one state to the other state, providing an indication of latch-up. Electrical circuits determine the position of the magnetic assembly, armature, within the magnetic latching mechanisms, and monitor voltage and current levels to provide the user with an indication of the operating condition of both the electronics and magnetic latching mechanisms. A separate electrical circuit provides a manual mode for operation of the magnetic latching mechanism.
An object of the present invention is to provide a power management system adapted for connection to a power source and an electromagnetic device wherein: the power management system determines at least one of a voltage, a current or a resistance; and wherein the system performs diagnostics on both the power management system and the electromagnetic device.
Another object of the present invention is to provide a power management system where the power management system includes a wave form generator to create a signal that is sent to the power management system and where the power management system senses the output of a position verification device.
Another object of the present invention is to provide a power management system where the position verification device detects a change in magnetic characteristics of the electromagnetic device.
In another aspect of the invention, an object of the present invention is to provide a power management system adapted for connection to a power source and an electromagnetic device where the power management system receives a signal from a switch requesting a change of state of the electromagnetic device; the electromagnetic device having a first state being an applied state and a second state being a release state; the power management system displaying a first indicia when the state of the electromagnetic device is changed from the first state to the second state and the power management system displaying a second indicia when the state of the electromagnetic device is changed from the second state to the first state.
Another object of the present invention is to provide a power management system where the power management system includes at least one parameter; the parameter being set to a failure indication, the power management system attempting to operate the electromagnetic device; the power management system sensing the operation of the electromagnetic device and if the operation of the electromagnetic device was successful, then the power management system setting the parameter to a completed cycle indication.
Another object of the present invention is to provide a power management system where the electromagnetic device further comprises a first state and a second state, a change from the first state to the second state being defined as a first change of state and a change from the second state to the first state being defined as a second change of state, where upon sensing a first change of state, the power management system prepares the power management system to anticipate a second change of state.
In another aspect of the present invention, an object of the present invention is to provide a power management system adapted for connection to a power source and an electromagnetic device where the power management system receives a control signal from a switch requesting a change in state of the electromagnetic device and wherein the power management system emits a first audible signal in response to the control signal.
Another object of the present invention is to provide a power management system where the power management system includes an electromagnetic device having a first state and a second state where the electromagnetic device has a first state being an applied state and a second state being a release state; the power management system emitting a second audible signal when the state of the electromagnetic device is changed from the first state to the second state and the power management system emitting a third audible signal when the state of the electromagnetic device is changed from the second state to the first state.
In another aspect of the present invention, an object of the present invention is to provide a power management system adapted for connection to a power source and an electromagnetic device where: a first control system receives a first control signal, and based on the control signal, sends electrical energy to an electromagnetic device; a second back-up control system, upon sensing a failure of the first control system, sends electrical energy to the electromagnetic device; and a manual override control system receiving a signal from a manual override enable switch, the manual override control system receiving a second control signal and based on the second control signal, sending electrical energy to the electromagnetic device.
Another object of the present invention is to provide a power management system where the first operating system and the second operating system operate independently from one another.
Another object of the present invention is to provide a power management system where the first operating system and the second operating system operate simultaneously.
Another object of the present invention is to provide a power management system where the failure of the first operating system is set as the default condition and if the default condition is met, the second operating system sends the electrical energy to the electromagnetic device.
In another aspect of the present invention, an object of the present invention is to provide a power management system adapted for connection to a power source and an electromagnetic device where: the power management system determines that the electromagnetic device is in a first state and that the electromagnetic device has become unseated; and upon determining that the electromagnetic device has become unseated, the power management system resets the electromagnetic device to the first state.
Another object of the present invention is to provide a power management system where the electromagnetic device produces a magnetic field, and wherein the power management system detects a current produced by the magnetic field to determine if the electromagnetic device has become unseated.
In another aspect of the invention, an object of the present invention is to provide a braking system for use in a vehicle comprising: an initiator for sending a control signal; a power management system, adapted for connection to a power source and a force generating system wherein the power management system controls energy delivered to the force generating system from the power source; a force transfer system connected to the force generating system, the force transfer system adapted to transfer force from the force generating system to a brake; where the braking system performs at least one diagnostic check.
Another object of the present invention is to provide a braking system for use in a vehicle comprising: an initiator for sending a control signal; a power management system, adapted for connection to a power source and a force generating system wherein the power management system controls energy delivered to the force generating system from the power source; a force transfer system connected to the force generating system, the force transfer system adapted to transfer force from the force generating system to a brake; where the braking system monitors at least one fault condition.
Additional features and advantages of the invention well be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practicing the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims as well as the appended drawings.